RFR 8209171 : Simplify Java implementation of Integer/Long.numberOfTrailingZeros() (original) (raw)

Ivan Gerasimov ivan.gerasimov at oracle.com
Mon Aug 13 17:10:26 UTC 2018

Hi Martin!

Good point about the command line flags, thanks!

These variants are close to numberOfTrailingZeros_07 that I've already tested, though you did better by saving one arithmetic operation at the return line!

I'll rerun the benchmarks.

With kind regards,

Ivan

On 8/13/18 7:56 AM, Martin Buchholz wrote: > The number of plausible variants is astonishing! >> --- >> Your use of -client and -server is outdated, which explains why you > get the same results for both (-client is ignored). >> I'm not sure what's blessed by hotspot team, but for C1 I > use -XX:+TieredCompilation -XX:TieredStopAtLevel=1 and for C2 I > use -XX:-TieredCompilation -server >> --- >> Now I understand the advantage of using ~i & (i - 1): the subsequent > zero check is a short-circuit for all odd numbers, better than i & -i, > which explains your results - they depend on being able to short-circuit. >> So just use a more faithful inlining of nlz without trying to improve > on it. >> static int ntzinlineNlz5(int i) { > i = ~i & (i - 1); > _if (i <= 0)_ > return (i == 0) ? 0 : 32; > int n = 1; > if (i >= 1 << 16) { n += 16; i >>>= 16; } > if (i >= 1 << 8) { n += 8; i >>>= 8; } > if (i >= 1 << 4) { n += 4; i >>>= 4; } > if (i >= 1 << 2) { n += 2; i >>>= 2; } > return n + (i >>> 1); > } >> But it's hard to resist the urge to optimize out a branch: >> static int ntzinlineNlz6(int i) { > i = ~i & (i - 1); > _if (i <= 0) return i & 32;_ > int n = 1; > if (i >= 1 << 16) { n += 16; i >>>= 16; } > if (i >= 1 << 8) { n += 8; i >>>= 8; } > if (i >= 1 << 4) { n += 4; i >>>= 4; } > if (i >= 1 << 2) { n += 2; i >>>= 2; } > return n + (i >>> 1); > } >

With kind regards, Ivan Gerasimov

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